1. Field of Invention
This invention relates to data recovery from recorded media such as magnetic tape. In particular, the invention relates to time base error correction and synchronization of information recorded in magnetic media.
There is a great need to accurately recover data which has been recorded on a magnetic medium in synchronism with a stable time base. For example, in capstan-driven servo-controlled instrumentation tape recorders which record information by frequency modulating a carrier recorded on a selection of tracks of a multiple-track tape, mechanical variations, such as tape dimension irregularities and capstan speed irregularities, may render it difficult to synchronize the information to a stable time base.
In particular applications, for example, in applications relating to archives of recorded tapes, the data recording system must conform to the constraints of the original recording. For example, certain U.S. Government archive recordings of instrumentation data have a format specification of two reference signals recorded on nonadjacent tracks of a twenty-eight track system. The two reference signal tracks are intended to be used to derive pilot signals for coherent decoding of up to twelve data signal tracks. In systems where two reference signals are employed, a physical skew arises between the reference signal tracks and the data signal tracks due to axial irregularities of the tape and differences in axial alignment between the record tape head of the source recorded and the reproduce tape head of the reproduce recorder. There is therefore a need to correct the skew error in order to coherently recover the desired data signals.
Time base errors also arise because of dynamic differential in speed between recording and reproduction such as wow and flutter.
2. Description of the Prior Art
Time base error correction is known to the art wherein analog signals are directly converted to digital values by conventional analog-to-digital converters, processed digitally and then converted back to analog signals by conventional digital-to-analog converters. Totally digital systems quite often require large amounts of digital memory and sophisticated high-speed processors. It is believed that the best signal-to-noise ratio achievable by known methods, either digital or analog, is between 60 and 70 dB. As a consequence, extremely low level signals, even though recorded, are lost in noise induced by the data recovery system.
It is therefore an object of the present invention to provide data recovery through a system having a signal-to-noise ratio substantially better than the signal-to-noise ratio available by known techniques.
It is also an object of the invention to recover data with a stable and accurately corrected time base.
It is a still further object of the invention to recover data with an accurate and stable time base using two reference signals recorded independently of the data signals on the same medium. These and other objects are accomplished in the invention described as follows.